1. Field of the Invention
The present invention relates to an apparatus for repeating the same process and more particularly to an apparatus for producing an automatic reciprocating motion of a movable body between two arbitrarily set positions.
2. Description of the Prior Art
The conventional apparatus of this kind usually uses an electric motor for moving a movable body. In order to repeat the reciprocal motion of the movable body, the polarity of the exciting coil of the motor is changed over by means of a change-over switch so as to rotate the motor in the forward and reverse direction.
On the other hand, the setting circuit which sets the two extreme points between which the movable body is automatically reciprocated, consists of two variable resistors for setting the two extremes and another variable resistor operated in gang with the motor.
As the movable body shifts, the voltage at the sliding contact of the variable resistor coupled to the motor varies continuously. This voltage is alternately compared with the voltages at the sliding contacts of the variable resistors for setting the extreme positions.
The two voltages at the sliding contacts of the position setting variable resistors are changed over by a change-over switch. It is when the two compared voltages are equal to each other, that is, the movable body reaches one of the extreme positions, that the change-over switch is actuated.
Moreover, whenever the change-over switch is actuated, the polarities of a power source connected with the two variable resistors for position setting must be switched over by another change-over switch operated in gang with the above change-over switch.
Usually, relays are used as such change-over switches as mentioned above. And these relays are controlled by a control circuit including a detecting circuit which checks the coincidence of the two compared voltages.
However, such a conventional apparatus as described above has the following drawbacks. First, in the conventional apparatus, there are used a plurality of change-over switches such as relays for changing over the polarities of the voltages at the sliding contacts of the variable resistors for position setting and the polarities of the power source connected with the variable resistors. (Hereinafter, only relays are referred to because relays are used as such change-over switches practically.) The use of the relays which are never free from the deterioration of contacts, will therefore shorten the life of the apparatus as a whole and also result in the degradation in reliability due to the malfunction of the worn-out contacts. In addition to this, the material for the contacts of the relays must be carefully selected since the current flowing through the sliding contacts of the variable resistors is interrupted by the contacts of the relays.
Secondly, at the start of operation, since the motor is rotated in the direction determined by the initial condition of the above mentioned control circuit, the movable body must be previously located between the two extreme positions. This lowers the efficiency of the operation of the apparatus.
Thirdly, since the change in the voltage at the sliding contact of the variable resistor coupled to the motor is not characterized by the direction of the movement of the movable body, the voltage does not represent the position of the movable body. Hence, even if the voltage is taken out and applied to the input terminal of the X-axis of an X-Y recorder, the recording in accordance with the position of the movable body cannot be performed.